1. Field of the Invention
The present invention relates to a cold-rolled high-tension steel for deep drawing suitable for use as the materials of automotive inner and outer panels. The steel has a ferrite single-phase structure, exhibits a tensile strength not lower than 40 kgf/mm.sup.2 and has excellent forming workability, as well as superior surface treatment characteristics. The invention also is concerned with a method for producing such a cold-rolled high-tension steel sheet.
2. Description of the Related Art
Cold-rolled steel steels have been used as materials of automotive parts such as structural members and outer panels. In particular, cold-rolled high-tension steel has been used as the material of such steel sheets in order to meet the requirement for reducing the weight of automobile. Important requisites for cold-rolled high-tension steels for use in automobiles are high forming workability, in particular press-workability, strength large enough to provide security of automobiles, and anti-secondary embrittlement characteristic which prevents embrittlement which may occur during secondary processing conducted after the forming work. In recent years, there is an increasing demand for rust prevention of steel sheets and, therefore, surface treating characteristics of the steel sheets are also becoming a matter of great significance.
Legal controls on total exhaust emissions from automotive engines are becoming more strict, which naturally requires reduction in weights of automobiles for reducing fuel consumption. In order to cope with such a demand, it is very important to develop light-weight and strong steel sheets.
Hitherto, various high-tension steel sheets having excellent workability have been proposed. For instance, Japanese Patent Laid-Open No. 57-181361 discloses a cold-rolled steel sheet which has a high Young's modulus and which is suitable for large-size works, as well as a method of producing such a steel sheet. Japanese Patent Laid-Open No. 58-25436 discloses a method of producing a cold-rolled steel sheet which is suitable for deep drawing and which has a high resistance to aging, as well as small anisotropy. These steel sheets are very-low-carbon steels containing a small amount of Nb and Ti and are produced through a continuous annealing conducted under specific conditions. These steels further contain P as reinforcement elements, in order to develop higher tensile strength.
The present inventors have conducted tests on several high-P steels having compositions similar to those shown in the above-mentioned Japanese Patent Laid-Open publications and found that such steels commonly exhibit a reduction in the mean Lankford value after cold-rolling and annealing, as well as inferior performance after painting.
Very-low-carbon steels having a high P content, in particular those having a C content less than 0.002 wt. %, exhibit tensile strength which is 40 kgf/mm.sup.2 at the highest, which is still too low to meet the requirements for steel sheets to be used as automotive parts having reduced weight and high strength.
Japanese Patent Publication No. 63-9579 discloses a high-strength cold-rolled steel sheet which contains, as a reinforcement element, Cu in addition to P and which exhibits high tensile strength not smaller than 40 kgf/mm.sup.2, as well as a high quality sheet surface. This steel sheet, however, still exhibits inferior surface treatment characteristics.